Reactions of Alkenes with Hydrogen
A concise revision guide to the reaction of alkenes with hydrogen, including the change from alkene to alkane, the use of a nickel catalyst and why this reaction is both an addition reaction and a reduction reaction.
The Overall Reaction
Alkenes react with hydrogen, H2, in the presence of a nickel catalyst. The reaction converts an alkene into an alkane.
The C=C double bond opens and one hydrogen atom adds to each carbon atom from the original double bond. This removes the alkene functional group and produces a saturated hydrocarbon.
For example, ethene reacts with hydrogen to form ethane.
Key idea: Hydrogen adds across the C=C double bond, so the alkene becomes an alkane.
Ethene reacts with H2; the C=C double bond is converted into a C-C single bond as ethane forms.
Why This Is an Addition Reaction
This reaction is an addition reaction because atoms are added across the carbon-carbon double bond.
The alkene starts with a C=C double bond, meaning it is unsaturated. During the reaction, the double bond opens and two new C-H bonds form.
The product is an alkane, which is saturated because it contains only single covalent bonds between carbon atoms.
Hydrogenation: The addition of hydrogen across a C=C double bond, converting an alkene into an alkane.
Remember: In alkene addition reactions, the C=C double bond is the reactive site. The double bond opens so new atoms can bond to the two carbon atoms.
Why This Is Also a Reduction Reaction
The reaction with hydrogen can also be described as a reduction reaction.
In organic chemistry, reduction often means the addition of hydrogen to a molecule. Since the alkene gains hydrogen atoms, the reaction is described as reduction.
This is why the same reaction can be correctly described in two ways: addition, because atoms add across the double bond, and reduction, because hydrogen is added.
Step 1: Hydrogen is the reagent
The alkene reacts with H2 in the presence of a nickel catalyst.
Step 2: The double bond opens
The C=C bond is replaced by a C-C single bond as new C-H bonds form.
Step 3: An alkane forms
The product is saturated because it no longer contains the alkene C=C functional group.
Exam focus: If asked for the reaction type, addition is essential. Reduction is also valid because hydrogen is added.
Common Exam Mistakes
Students often lose marks by forgetting the nickel catalyst, or by describing the reaction as substitution. This is not substitution because no atom is replaced.
Another common error is to leave the product as an alkene. The product must be an alkane, because the carbon-carbon double bond has been removed.
For a simple reaction question, the safest answer should include the reagent, condition, product type and reaction type.
Exam focus: Link the functional group change to the reaction type: alkene to alkane happens because hydrogen adds across the C=C double bond.
Check Your Understanding
Use these short activities to check the key ideas before moving on to the next alkene reaction.
Master Alkenes for Edexcel A Level Chemistry
Continue from these free revision notes into the full Topic 6C Alkenes course, with guided video teaching, diagnostic MCQ practice, teacher-marked short-answer questions and a specification assignment with a personalised progress report.
Guided video teaching
Learn the chemistry and exam technique through structured video lessons with worked examples and walkthroughs.
Instant MCQ feedback
Auto-marked MCQ quizzes provide immediate diagnostic feedback for every answer choice.
Teacher-marked SAQs
Submit written exam responses and receive chemistry specialist feedback with improvement guidance.
Progress tracking
Identify strengths and weaknesses across the full Topic 6C specification with targeted reporting.
See how the course works
Click play to start the course preview animation.
Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.